Directory UMM :Data Elmu:jurnal:S:Soil Biology And Chemistry:Vol32.Issue10.Sep2000:

Short communication

Carbon dioxide emission related to chemical properties of a

tropical bare soil

N. La Scala Jr.*, J. Marques Jr., G.T. Pereira, J.E. CoraÂ

FCAV-UNESP, Via de acesso Prof. Paulo Donato, Castellane s/n, 14870-000 Jaboticabal, SP, Brazil

Received 7 June 1999; received in revised form 5 January 2000; accepted 23 February 2000

Abstract

In this work the relationship between CO2emissions and the soil properties of a tropical Brazilian bare soil was investigated.

Carbon dioxide emissions were measured on three di€erent days at di€erent soil temperature and the soil moisture conditions, and the soil properties were investigated at the same points that emissions were measured. The soil CO2 emissions were

correlated to carbon content, cation exchange capacity and free iron content at the 65 points studied in an area of 100100 m

located in southern Brazil.72000 Elsevier Science Ltd. All rights reserved.

Keywords:Soil respiration; Soil CO2emission; Soil properties; Tropical soil properties

Studies on CO2 emissions from tropical soils have mainly investigated the relationship between emissions, soil temperature and soil moisture (Meir et al., 1996). As soils di€er in their characteristics, it is expected that soil CO2 emissions also di€er according to par-ticular properties of the soil, but little study has been done so far on the relationship between CO2emissions and tropical soil properties. However, the variability of CO2 emissions in soils covered with vegetation has been described as a simple multiple regression model, incorporating some soil properties as predictor factors (Fang et al., 1998; Rout and Gupta, 1989; Carlyle and Than, 1988). In this work we have investigated the CO2 emissions of a tropical bare soil and correlated the emissions with the soil properties, some of them being tropical soil properties, like iron level, for instance.

The study was conducted on a bare dark red latosol (oxisol) at FCAV-UNESP (21815'220 south 48818'580

west), SaÄo Paulo State: Brazil. A grid containing 65 points was established on the experimental site (100

100 m) where the points were spaced at distances of 20 and 10 m (Fig. 1). CO2 emissions from the soil were measured at each grid point on three di€erent days (19th, 25th and 27th November 1998) at di€erent soil temperatures and soil moisture conditions. Soil tem-peratures at 20 cm depth were measured at each grid point using LI-6400 Soil Temperature Probe (built by LI-COR, NE USA) at the same time the measure-ments of CO2emissions were taken. Soil samples were obtained from 0 to 20 cm depths on each grid point, on 27th November. These 65 samples were used for determining cation exchange capacity (CEC), total car-bon, pH, V% and sum of bases (Ca2+ + Mg2+ + K+) (Raij et al., 1987), texture (Gee and Bauder, 1986), gravimetric water content (Gardner, 1986) and free iron oxide: Fed (citrate ditionite bicarbonate extraction ) (Mehra and Jackson, 1960). Particle size distribution was determined by the pipette method, after dispersion of soil by adjusting the pH to 10±11 with 1 M NaOH, and sand sieving. Bulk density was calculated from the weight of the oven-dry mass and volume of the soil core. For chemical analyses, samples of the ®ne earth fraction (soil < 2 mm) were extracted with 1 M KCl for Ca2+, Mg2+ and Al3+, with 0.05 M HCl and 0.025 M H2SO4 for K+ and Na+, and

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with pH 7.0 1 M Ca(OAc)2for extractable acidity (H+ + Al3+). Extractable H+ was calculated by subtract-ing the titrated Al from extractable acidity. Cation Exchange Capacity (CEC) was calculated from the sum of base cations (Ca2+, Mg2+, K+ and Na+) plus extractable acidity. The total carbon was determined by wet oxidation with K2Cr2O7, according to Raij et al. (1987).

The CO2 emissions were measured using a 6400-09 soil CO2 ¯ux chamber built by LI-COR, NE USA (Healy et al., 1996). The chamber is coupled to a LI-6400 Photosynthesis system that computes the emis-sions coming from the soil to the chamber. Following each measurement mode, the ¯ux is calculated using the best ®t of a linear regression, which computes sev-eral CO2 ¯ux measurements from the soil to the chamber. The system operation avoids over pressures of CO2 inside the chamber by operating between a maximum and minimum CO2concentrations. The rate of increase in CO2concentration inside the chamber is monitored and the soil CO2 emission is computed when the chamber CO2 concentration is equal to that at the soil surface in the open. In all the measurements done for 3 days, a short sampling period of 1.5 min at each grid point was used to complete the sampling from the whole 65 points as quickly as possible and to avoid soil temperature variation in the grid during this period (Table 1).

Fig. 1 presents the post maps of CO2emission from the soil on the area described before. On comparing the maps, the CO2 emission values were found higher on 27th November and quite similar on 19th and 25th. The CO2 emission values varied from a minimum of 0.23 mmol mÿ2

sÿ1

, measured on 19th November, to a maximum of 5.80 mmol mÿ2 _sÿ1_{, measured on 27th} November (Table 1). The higher CO2 emission on 27 could be attributed to the di€erences of soil tempera-ture and soil moistempera-ture conditions on that referred day, because of a precipitation event (14.6 mm) registered on the night before. The studies by Howard and Howard (1993) and Singh and Gupta (1977) have also shown a strong dependence soil CO2emissions of soil CO2emissions on soil temperature and soil moisture.

Table 1 also presents the descriptive statistics of the physical and chemical soil properties and soil CO2 emissions on the three di€erent days. Based on the mean and standard deviation values, a high CO2 emis-sion variability (between 30% and 43%) was found in all the measurements for the 3 days. However, it is im-portant to notice that the emissions included in this study have variability values that are smaller than the ones reported in soils having vegetation cover (Fang et al., 1998; Rout and Gupta, 1989; Rochette et al., 1991). The chemical and physical properties have variability between 4% and 18%; soil temperature and

soil moisture have low variability in the grid studied (<5%).

In order to understand the individual contribution of soil properties to the soil CO2 emissions we have performed the linear correlation between them. Our results showed no linear correlation of temperature and gravimetric water content with CO2 emissions in all the measurements for the 3 days. However, CO2 emissions were linearly correlated (P< 0.05) to total carbon, CEC and Fedfor the measurements of all days (Fig. 2). The linear coecients (r) among F19, F25 and F27 and total carbon were 0.47, 0.35 and 0.30, re-spectively; that among F19, F25 and F27 and CEC were 0.39, 0.33, 0.27, respectively; and that among F19, F25 and F27 and Fed were ÿ0.22, ÿ0.36 and ÿ0.42, respectively.

In order to examine in more detail our experimental results, we have also applied a multiple linear re-gression (P < 0.05), trying to ®t the CO2 emissions F19, F25 and F27 as function of the soil properties studied. The results of a stepwise regression procedure are presented in Table 2. For F19, we have found that 30% of CO2 emission variability was explained by a linear combination of total carbon and sum of bases (SB). In the case of F25, 19% of CO2 emission varia-bility was explained by combination of Fed and total carbon. For F27, 18% of CO2emission variability was explained only by Fed. The parameters estimated for total carbon and SB are positive and the parameters estimated for Fed are negative, as indicated in the simple linear correlation analysis presented before. Therefore, the results obtained by the multiple linear regression is in accordance with the individual linear regression presented in Fig. 2. On 19th November,

total carbon was the property that contributed most to CO2 emissions, but on 25th and 27th November, the variable that have most contributed to the CO2 emis-sion was the Fedlevel.

The relationship of CO2 emissions with some of these soil characteristics is expected and it has been described in literature. Carbon is known as the basic element used by bacteria during the decomposition process (Singh and Gupta, 1977). Also, the CEC and carbon are dominant factors in soil respiration pro-cesses due to the fact that they keep pH values ade-quate for bacterial growth (Stotzky and Rem, 1966). Despite the in¯uence of CEC on the respiration pro-cesses in soil has been reported before, this study showed that such in¯uence is signi®cant even when computing CO2 emissions in a relatively small area (100100 m) at di€erent temperatures and soil moist-ure conditions.

Also, the results of this study showed signi®cant

Table 1

Descriptive statistics of values of soil CO2emission, soil temperature and soil properties measured in three di€erent days on 65 points studied a

Mean Minimum Maximum Standard deviation Coecient of variation (%)

F19 (mmol mÿ2

V% 40.69 27.00 60.00 6.85 16.8

Total carbon (g kgÿ1_{) 12.25 9.00 16.00 1.52 12.4}

Soil moisture (wt%) 22.42 19.42 24.61 1.21 5.4

Clay (g kgÿ1_{) 580.00 510.00 620.00 22.87 3.9}

F19 soil CO2emission on 19th November 1998. F25 soil CO2 emission on 25th November 1998. F27 soil CO2emission on 27th November 1998. T19 soil temperature on 19th November 1998. T25 soil temperature on 25th November 1998. T27 soil temperature on 27th November 1998.

Table 2

Results of stepwise regression of CO2emissions and the soil proper-ties studied

Measurement days Variables Parameter estimate P-value R2a

F19 Intercept ÿ1.79 0.0089

Carbon 0.19 0.0002

negative linear correlation of CO2 emissions with soil-free iron content. One possible explanation for this e€ect, as pH values are lower than 7 in our soils, is that the soil-free iron content is related with an increase of anion exchange capacity and a decrease in microbial activity (Bohn et al., 1985).

The results have shown that CO2 emissions in soil increases with carbon content and CEC and they decreases with soil-free iron content. This study suggests a more complex relationship between clay minerals and biological activity, including soil iron level as an important factor in order to infer the eco-logical impact of tropical soil management in the bio-sphere. Due to the distinguishable characteristics of the Brazilian soils in terms of iron level, there is need for performing additional experiments to elucidate the relationship of iron content and CO2 loss in di€erent classes of soils.

Acknowledgements

We are grateful to FAPESP and CNPq for ®nancial support.

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